Conventional Voltage-Controlled Oscillator (VCO) circuits are used to generate clock signals used in a variety of electronic circuits. In digital systems, VCOs are often used in frequency synthesizer phase-lock loop (PLL) circuits. PLL circuits use a feedback loop to provide an input voltage to a VCO that generates a stable output clock signal having a frequency that is an accurate and known multiple of a system reference clock frequency. Still other circuits, such as FM radio receivers, use a VCO-based PLL arrangement to demodulate an incoming frequency-modulated (FM) radio signal.
VCOs are widely used and perform critical functions in both digital and analog electronic systems. VCOs used in electronic circuits are often designed to have the output frequency as a linear function of input control voltage where FOUT=KVCO*VCTRL+F_OFFSET, where F_OFFSET represents a constant offset frequency the VCO will generate when a control signal VCTRL is zero volts. KVCO is known as the VCO gain. A low gain is desirable in low-jitter applications. For a given noise on the input voltage, VCTRL (the corresponding change in output frequency) is ΔFOUT=KVCO*ΔVCTRL.
To be useful in a wide variety of applications, a VCO should be able to generate a wide range of output frequencies (i.e., from several megahertz to tens of gigahertz). However, having a low-gain can be at odds with the ability to generate a wide range of output frequencies since the input voltage range is typically limited by the supply voltage and/or other circuit bias constraints.
VCOs used in electronic circuits are often designed so the period of the output has the lowest possible variation in the output period (also known as period jitter) when operating at a stable input voltage. The output is often designed to have an accurate duty-cycle close to 50%. VCOs are also designed to function over a large temperature range. A wide operating temperature range specification is often difficult to meet since the VCO is constructed from temperature-variant devices, such as transistors and resistors, that have properties which vary widely with temperature. A conventional VCO is also often designed to tolerate voltage supply noise and tolerate large variation in device process parameters (i.e., resistor resistivity, transistor turn-on voltage, etc.).
Referring to FIG. 1, a circuit 10 is shown implementing a conventional VCO. The circuit 10 shows a circuit 12 and a circuit 14. The circuit 12 is shown as a voltage-to-current converter (also referred to as a V-to-I or transconductance or Gm block). The circuit 14 is shown as a current-controlled oscillator (ICO). A Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET or MOS transistor) is often used as a transconductance device. The behavior of such a device can be characterized by the equation: I=K(Vgs)2, where Vgs is the voltage between the gate terminal and the source terminal of the device, and K is a coefficient which depends on device dimensions, temperature, voltages on the bulk and drain terminals, construction material and fabrication details. ICOs are often made of a number of stages connected in a ring fashion. Single-ended ICOs are often made with an odd number of stages.
It would be desirable to implement a VCO that performs reliably, predictably and accurately over a wide temperature and/or voltage range using readily-available fabrication processes such as CMOS.